1. Field of the Invention
The present invention relates to machines for the injection molding of plastic or metallic parts and articles, and, more particularly, to die closing units of injection molding machines which hold a multi-plate injection molding die, opening and closing the latter hydraulically by means of pull-type hydraulic actuators whose piston rods reach over or through the injection molding die.
2. Description of the Prior Art
Injection molding machines with pull-type die closing units are known in a number of versions and design variations. Generally, such a die closing unit has a stationary die carrier plate attached to, or integrally combined with a cylinder mount which accommodates two or four hydraulic linear actuators. The latter have parallel piston rods which move in unison and extend at right angles to the stationary die carrier plate, the far ends of the piston rods reaching a distance beyond said plate. To the far ends of the piston rods is rigidly attached a die carrier plate which is movable towards and away from the stationary die carrier plate. The two plates have suitable die mounting faces against which can be mounted the constituent plates of a multi-plate injection molding die. The action of the linear actuators of the die closing unit, by moving the movable die carrier plate and attached die half, opens and closes the injection molding die.
This type of die closing unit may be subject to undesirable bending conditions at the long piston rods, especially when only two linear actuators are used, or when the space requirements of the injection molding die and a long opening stroke necessitate very long piston rods, even when four linear actuators are used. In order to eliminate these bending conditions, it is therefore common practice to provide a stationary support for the movable die carrier plate in the form of a cantilever frame which is rigidly mounted to the machine base. The movable die carrier plate engages this guide frame with suitable guide rollers. Such a guide frame, in addition to increasing the complexity and cost of the die closing unit, has the disadvantage of restricting the accessibility of the injection molding die from underneath.
A die closing unit of the type described above is disclosed in U.S. Pat. No. 4,025,264. This prior art structure features four linear actuators having piston rods which reach through all the plates of the die closing unit, the far ends of the piston rods being attached to a movable die carrier plate which is supported on a stationary cantilever guide frame. In this prior art die closing unit, the injection molding die consists of two stationary die plates, of which one is a mounting plate and the other a mold plate, and two movable die plates, of which one is likewise a mold plate and the other serves as an ejector plate. The four plates, plus the movable die carrier plate to which the ejector plate and movable mold plate are attached, require rather long piston rods which would deflect under the weight of the plates, without a suitable stationary support for the movable die carrier plate.
The prior art die closing unit which is disclosed in U.S. Pat. No. 4,025,264 also features oversize die plates which embrace the four piston rods with guide bores, thereby being centered on the piston rods. The primary advantage of these oversize die plates is that they make it possible to arrange a larger mold cavity in the injection molding die. However, the use of oversize die plates is only then possible, when the piston rods of the die closing unit are retractable in the axial direction, for the purpose of inserting and removing the die plates. The oversize die plates also have the additional advantage of providing a degree of support for the extended piston rods, the guide bores of the mounting plate and of the stationary mold plate providing an axial extension of the piston rod support which would otherwise end at the stationary die carrier plate, where the piston rods emerge from the linear actuators. Yet, even this extension of the piston rod support to the separation plane of the injection molding die cannot entirely eliminate the need for a stationary support frame underneath the movable die carrier plate, especially in the case of heavy-duty injection molding machines, or in the case of machines which require a great amount of space for the injection molding die and a long opening stroke.